Polyester resins are extensively used in various applications including containers, e.g., bottles, films, sheets, and fibers because they have excellent chemical and physical properties.
In general, a polyester resin is produced through the esterification reaction or transesterification reaction of a dicarboxylic acid and/or an ester-forming derivative thereof with a diol and a melt polycondensation reaction. According to need, the resultant polymer is further subjected to a solid-phase polycondensation reaction especially in producing a resin for container use. For these polycondensation reactions, a compound of antimony, germanium, titanium, or the like is used as a catalyst.
However, polyester resins produced with an antimony compound as a catalyst have peculiar dullness. Because of this and because there is a fear about safety/hygiene and the environment in the use of antimony compounds, there has been a strong desire for a reduction of the amount of the antimony compound to be used or for the advent of a polycondensation catalyst usable as a substitute for the antimony compound. Polyester resins produced with a germanium compound as a catalyst are satisfactory in transparency, safety/hygiene, and the like but unavoidably have a disadvantage in cost because the germanium compound itself is exceedingly expensive. From this and other standpoints, there has been a strong desire, in the case of germanium compounds also, for a reduction in the amount of the germanium compound to be used or for the advent of a polycondensation catalyst usable as a substitute for the compound.
On the other hand, titanium compounds are attracting attention because they are inexpensive and are free from a fear about safety/hygiene, etc. Titanium compounds have come to be used in place of antimony compounds or germanium compounds. However, polyester resins produced with a titanium compound as a catalyst have peculiar yellowness and have further had quality defects such as poor thermal stability.
For overcoming such defects, patent document 1 proposes a process in which titanium, magnesium, and phosphorus compounds are separately added as respective ethylene glycol solutions in specific proportions to thereby produce a polyester resin of excellent quality. In this process, however, at least three independent catalyst addition devices are necessary and it is not easy to divert an existing polyester resin production equipment employing an antimony catalyst or the like to this process. It is therefore necessary to additionally install a new apparatus, resulting in the necessity of a further investment.
There has hence been a desire for a polycondensation catalyst for polyester production which contains titanium atoms, atoms of an alkaline earth metal such as magnesium, and phosphorus atoms in the same catalyst and which enables an existing polyester resin production equipment to be stably used as it is.
Patent document 2 discloses, as such a polycondensation catalyst for polyester production, a catalyst comprising an alkyl titanate treated with an aromatic polycarboxylic acid, a magnesium compound, a specific phosphorus compound, and ethylene glycol. However, as a result of an investigation made by the present inventors, this catalyst was found to lack long-term stability in ethylene glycol and opacify in several days during storage to give a slurry containing a precipitate of a metal ingredient. Since this catalyst is in a slurry form, the catalyst was found to have problems concerning poor handleability. For example, it is difficult to add the catalyst with satisfactory precision during a polycondensation reaction for polyester production, and the catalyst in a slurry form sediments in a catalyst storage tank or catalyst addition line and scales. Furthermore, there has been a problem that since it is essential to treat a titanium compound with an aromatic polycarboxylic acid for preparing the catalyst, this makes the production steps complicated and more costly. In addition, this process necessitates a heating step for heating to 100° C. or higher and, hence, is not easy to practice.
Patent document 3 discloses a homogeneous-solution catalyst comprising ethylene glycol or the like and, contained therein, titanium atoms and atoms of a Group 2A metal of the periodic table. In patent document 3, it is disclosed to add an organic acid or inorganic acid to the solution to make the solution acidic for the purpose of keeping the turbidity of this homogeneous-solution catalyst stably low. However, there is no statement in this patent document concerning a technique in which the phosphorus compound to be added in a polyester resin production reaction is added to the homogeneous solution. In the process proposed, it is necessary in polyester resin production to separately add the phosphorus compound as a stabilizer for the polyester resin.
In the processes according to patent documents 1 and 3, a solution containing titanium element, magnesium element, and phosphorus element is added in two or three portions. However, in the case of employing portion-wise addition in industrial production, it is necessary to strictly control an addition method and the like in order to enable the polycondensation catalyst components to be present in the reaction system in given concentrations from the standpoints of productivity or stability of the performance and product quality of the polyester to be obtained, etc.
Patent document 4 discloses a process employing as a catalyst an ethylene glycol solution containing titanium atoms and magnesium atoms which has been obtained using, as a raw material, a titanium compound synthesized beforehand which is a solid titanium compound or a solid titanium compound containing magnesium. However, this technique has a problem that the catalyst production steps are complicated partly because of the necessity of a step in which a metal compound is hydrolyzed and that the product is costly accordingly. Furthermore, there is no statement in this patent document concerning a technique in which the phosphorus compound to be added in a polyester production reaction is added to this ethylene glycol solution, and it is necessary to separately add the phosphorus compound during polyester production.
For overcoming such drawbacks, patent document 5 proposes: a liquid polycondensation catalyst for polyester production which contains titanium atoms, alkaline earth metal atoms, and phosphorus atoms, can be easily produced industrially, is less expensive, and has excellent long-term storage stability; and a process for producing the catalyst. However, the liquid catalyst of this invention contains titanium atoms in a relatively low concentration and is hence bulky. Because of this and because the catalyst contains a solvent in a large amount, the liquid catalyst proposed is unsuitable for conveyance/transport.    Patent Document 1: JP-A-2004-124068    Patent Document 2: JP-A-2004-224858    Patent Document 3: JP-A-2004-292803    Patent Document 4: JP-A-2004-176033    Patent Document 5: Japanese Patent Application No. 2005-014379